I went walking in the woods in Sharon, Vermont, the other day, wearing shorts and sandals – which would have been pretty risky in Connecticut, where I grew up. Lacking protective, insecticide-saturated, tucked-in armor, I would have been the naked lunch of a black-legged tick, commonly known as the deer tick, carrier of Lyme disease, whose bite, if left untreated, can cause severe arthritis or neurological problems.
We are fortunate to be living in a place where Lyme disease is not yet common – but we are not out of the woods. All New Hampshire counties have reported cases, and the two southeastern New Hampshire counties are ranked in the top tenth percentile of cases by county nationwide. In Vermont’s southern four counties, 9.7 percent of 92 dogs tested in a 2000-2001 study had Lyme disease. According to Patsy Tassler, epidemiologist at the Vermont Department of Health, “If dogs are getting Lyme disease in Vermont, it follows that people can get it here too.”
The deer tick’s life cycle affects the timing and severity of Lyme disease outbreaks in human populations. Adult ticks are carried into oak forests, usually by deer, and especially after a bumper crop of acorns (usually every two to five years.) Adult ticks drop off the deer and spend the winter in the leaf litter of the forest floor.
The following spring, adult females lay eggs that hatch into larval ticks. Research shows that a high density of ticks in one summer corresponds closely with a large acorn crop the autumn before. Larval ticks are not infected with Lyme disease when they hatch; instead, they seek out blood meals, usually white-footed mice, whose populations are also large – sometimes a hundred times more mice than in the year before – due to the previous year’s large acorn crop.
White-footed mice are a common carrier for Lyme disease and the primary transmitters of the disease to larval ticks. Ticks that contract Lyme disease from their mouse hosts drop to the forest floor, molt into nymphs, and overwinter. The following spring, the infected nymphs seek out another host, often a human passing through the forest. The end result: cases of Lyme disease in humans are most common two summers after a bumper crop of acorns.
Humans contract Lyme disease when a deer tick’s bite releases Borellia burgdorferi bacteria into our bloodstream. These ticks are smaller than a pinhead, and thus extremely hard to spot. Infection usually occurs about one day after the bite. Without quick antibiotic treatment, victims can suffer from chronic back, muscle, and joint pain for the rest of their lives. But if a tick is quickly removed and no bulls-eye rash develops where it was stuck, chances of infection are low.
One twist on this chain of events is that gypsy moth outbreaks reduce the spread of Lyme disease. Cyclical gypsy moth outbreaks can defoliate hundreds of acres of oak forests. The year after defoliation, oak trees are less likely to put out large acorn crops. With fewer acorns, fewer white-footed mice survive, and thus fewer ticks with Lyme disease, leading to fewer human infections. But eventually the trees recover and produce more acorns, resulting in more mice, which prey on gypsy moth pupae, and the balance tips back in favor of Lyme disease for a year or two.
Meanwhile, people are inadvertently creating more tick habitat. When forests are cleared for development, leaving fragments of five acres or less, the risk of humans contracting Lyme disease is seven times greater than it would be in larger blocks at least in part because white-footed mice thrive in these small patches while their predators and other tick hosts do not. With a surplus of mice compared to other tick hosts, like opossums, more deer ticks will find their blood meals from the mice – lots of mice, at that – and thus be at a greater risk of contracting Lyme disease.
So, are we to don our protective gear and accept the inevitability of Lyme disease moving into our backyards? It may not be clear yet, but rest assured, there are experts working on an answer, including Dr. Rick Ostfeld, an ecologist at the Institute for Ecosystem Studies in Millbrook, New York. “The lesson is clear,” he says. “Oak trees, mouse populations, and forest destruction all can influence risk. Our challenge is to figure out how much of this enormous complexity we need to understand.” For more information on Lyme disease prevention and diagnosis, visit www.lymenet.org.